Absolute encoder

ABSTRACT

To reduce the influence of an unintended magnetic flux of a permanent magnet on detection accuracy. An absolute encoder ( 2 ) includes a magnet provided at a tip end side of any one of a first worm gear portion ( 11 ) and a second worm gear portion ( 22 ), and an angle sensor configured to detect a rotation angle of the magnet in response to a change in magnetic flux generated from the magnet. In the magnet, a first magnetic pole portion of a first polarity and a second magnetic pole portion of a second polarity different from the first polarity are formed adjacent to each other as viewed from an axial end surface of the magnet. The first magnetic pole portion and the second magnetic pole portion are formed adjacent to each other in a radial direction with a radial center of the magnet as a boundary, and the first magnetic pole portion and the second magnetic pole portion are formed adjacent to each other in an axial direction with an axial center as a boundary. The absolute encoder ( 2 ) is provided with a magnetic interference reduction member formed of a magnetic material on a radially outer peripheral surface of the magnet.

TECHNICAL FIELD

The present invention relates to an absolute encoder.

BACKGROUND ART

Conventionally, a rotary encoder is known to be used to detect theposition and angle of a moving element in various types of controlmachines. Such a rotary encoder includes an incremental-type encoderdetecting a relative position or angle, and an absolute-type absoluteencoder detecting an absolute position or angle. As such an absoluteencoder, there is known a magnetic encoder device configured to detectan amount of rotation of a main shaft to be measured by way of attachinga magnetized permanent magnet to a rotating shaft (main shaft) to bemeasured and detecting the angle of rotation of the permanent magnet byusing a magnetic sensor. There is also known a method for identifyingthe amount of rotation of the main shaft over multiple rotations byacquiring the rotation angle of a rotating body rotating anddecelerating according to the rotation of the main shaft.

For such an absolute encoder, for example, a structure including amagnet with the N-pole and the S-pole arranged in a circumferentialdirection of a rotation axis line in a magnet accommodating portion of amagnet holder has been proposed (see, for example, Patent Literature 1).

CITATION LIST Patent Literature

Patent Literature 1: JP 2018-72086 A

SUMMARY OF INVENTION Technical Problem

In an absolute encoder detecting the amounts of rotation of a pluralityof permanent magnets using such a reduction mechanism, when theplurality of permanent magnets and a magnetic rotation angle sensor areprovided, the accuracy of the magnetic rotation angle sensor maydecrease when the magnetic rotation angle sensor detects unintendedmagnetic flux of the permanent magnets. Therefore, in order to improvethe detection accuracy, this type of absolute encoder is required tohave a structure capable of eliminating the influence of an unintendedmagnetic flux of the permanent magnets.

The present invention has been made in view of the above problems, andan object thereof is to provide an absolute encoder capable of reducingthe influence of an unintended magnetic flux of a permanent magnet ondetection accuracy.

Solution to Problem

In order to achieve the above object, an absolute encoder configured toidentify an amount of rotation of a main shaft over multiple rotationsincludes: a first drive gear configured to rotate according to rotationof the main shaft; a first driven gear configured to mesh with the firstdrive gear; a second drive gear provided coaxially with the first drivengear and configured to rotate according to rotation of the first drivengear; a second driven gear configured to mesh with the second drivegear; and a magnet provided at a tip end side of at least one of thefirst driven gear and the second driven gear, and an angle sensorconfigured to detect a rotation angle of the first driven gear or thesecond driven gear provided with the magnet in response to a change in amagnetic flux generated from the magnet, in which a first magnetic poleportion of a first polarity and a second magnetic pole portion of asecond polarity different from the first polarity are formed adjacent toeach other in the magnet as viewed from an axial end surface of themagnet, the first magnetic pole portion and the second magnetic poleportion are formed adjacent to each other in a radial direction with acenter of the magnet in the radial direction as a boundary, the firstmagnetic pole portion and the second magnetic pole portion are formedadjacent to each other in the axial direction with a center in the axialdirection as a boundary, and a magnetic interference reduction memberformed of a magnetic material is provided on an outer peripheral surfaceof the magnet in the radial direction.

Advantageous Effects of Invention

With the absolute encoder according to the present invention, it ispossible to reduce the influence of an unintended magnetic flux of apermanent magnet on detection accuracy.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view schematically illustrating a configurationof an absolute encoder according to an embodiment of the presentinvention.

FIG. 2 is a perspective view schematically illustrating theconfiguration of the absolute encoder in FIG. 1 with a case removed.

FIG. 3 is a perspective view schematically illustrating theconfiguration of the absolute encoder in FIG. 2 with a substrate and aconnector removed.

FIG. 4 is a perspective view schematically illustrating theconfiguration of the absolute encoder in FIG. 3 when viewed from anotherangle.

FIG. 5 is a perspective view schematically illustrating theconfiguration of the absolute encoder in FIG. 3 with a motor removed.

FIG. 6 is a plan view schematically illustrating the configuration ofthe absolute encoder in FIG. 5 .

FIG. 7 is a cross-sectional view of the absolute encoder in FIG. 1sectioned along a plane parallel to the central axis of a main shaft.

FIG. 8 is a cross-sectional view schematically illustrating theconfiguration of the absolute encoder in FIG. 1 with the motor removed,sectioned along a plane through the central axis of a main shaft gearand orthogonal to the central axis of an intermediate gear.

FIG. 9 is an enlarged cross-sectional view schematically illustratingportions around a magnet and a magnetic interference reduction member inthe configuration of the absolute encoder in FIG. 8 .

FIG. 10 is an exploded longitudinal cross-sectional view schematicallyillustrating configurations of the magnet, the main shaft gear, a mainshaft adapter, and a main shaft of the motor in the configuration of theabsolute encoder in FIG. 8 .

FIG. 11 is a cross-sectional view schematically illustrating theconfiguration of the absolute encoder in FIG. 6 , sectioned along aplane through the central axis of the intermediate gear and parallel toan XY plane.

FIG. 12 is an enlarged perspective view of the cross-sectional view inFIG. 11 as viewed from another angle.

FIG. 13 is a partial cross-sectional view schematically illustrating theconfiguration of the absolute encoder in FIG. 6 , sectioned along aplane through the central axis of the intermediate gear and orthogonalto the XY plane.

FIG. 14 is an exploded perspective view schematically illustrating theconfiguration of the absolute encoder in FIG. 12 with a base, theintermediate gear, an intermediate gear shaft, a leaf spring, and ascrew disassembled.

FIG. 15 is a partial cross-sectional view schematically illustrating theconfiguration of the absolute encoder in FIG. 2 , sectioned along aplane through the central axis of a secondary shaft gear and orthogonalto the central axis of the intermediate gear.

FIG. 16 is an exploded perspective view schematically illustrating theconfiguration of the absolute encoder in FIG. 15 with the magnet, amagnet holder, the secondary shaft gear, and a bearing disassembled.

FIG. 17 is a diagram schematically illustrating a modified example of asupport projection supporting the main shaft-side end portion of theintermediate gear shaft in the absolute encoder.

FIG. 18 is a diagram schematically illustrating a modified example ofthe support projection supporting the main shaft-side end portion of theintermediate gear shaft in the absolute encoder.

FIG. 19 is a diagram schematically illustrating a modified example ofthe support projection supporting the main shaft-side end portion of theintermediate gear shaft in the absolute encoder.

FIG. 20 is a view of the substrate in FIG. 2 as viewed from a lowersurface side.

FIG. 21 is a block diagram schematically illustrating a functionalconfiguration of the absolute encoder in FIG. 1 .

DESCRIPTION OF EMBODIMENTS

The present inventor found that, in an absolute encoder, an amount ofrotation of the main shaft over multiple rotations (hereinafter, alsoreferred to as “multiple rotations”) (hereinafter, also referred to as“amount of rotation of the main shaft”) can be identified by acquiring arotation angle of a rotating body rotating and decelerating according tothe rotation of the main shaft. That is, the amount of rotation of themain shaft can be identified by multiplying the rotation angle of therotating body by a reduction ratio. Here, the range of the identifiableamount of rotation of the main shaft increases in proportion to thereduction ratio. For example, if the reduction ratio is 50, the amountof rotation of the main shaft over 50 rotations can be identified.

On the other hand, the required resolution of the rotating bodydecreases in proportion to the reduction ratio. For example, if thereduction ratio is 100, the resolution required for the rotating bodyper rotation of the main shaft is 360°/100=3.6°, and a detectionaccuracy of ±1.8° is required. On the other hand, when the reductionratio is 50, the resolution required for the rotating body per rotationof the main shaft is 360°/50=7.2°, and a detection accuracy of ±3.6° isrequired.

Embodiments of the present invention will be described below withreference to the drawings. In each of the embodiments and modifiedexamples described below, the same or equivalent components and membersare denoted by the same reference symbols, and redundant descriptionsare omitted as appropriate. The dimensions of the members in eachdrawing are enlarged or reduced as appropriate to facilitateunderstanding. Furthermore, some members that are not critical indescribing embodiments are omitted from the drawings. Also, in thedrawings, gears are illustrated without a gear shape. Terms includingordinal numbers such as “first” and “second” are used to describevarious components, but these terms are used only for the purpose ofdistinguishing one component from other components and the componentsare not limited by these terms. Note that the present invention is notlimited by the embodiments described below.

An absolute encoder 2 according to an embodiment of the presentinvention is an absolute encoder configured to identify an amount ofrotation of a main shaft la over multiple rotations. The absoluteencoder 2 includes a first worm gear portion 11 as a first drive gearrotating according to the rotation of the main shaft 1 a, and a firstworm wheel portion 21 as a first driven gear meshing with the first wormgear portion 11. The absolute encoder 2 further includes a second wormgear portion 22 provided coaxially with the first worm wheel portion 21and serving as a second drive gear rotating according to the rotation ofthe first worm wheel portion 21, and a second worm wheel portion 31 as asecond driven gear meshing with the second worm gear portion 22. Theabsolute encoder 2 includes magnets Mp and Mq provided on the tip endside of at least one of the first worm gear portion 11 and the secondworm gear portion 22, and angle sensors Sp and Sq for detecting arotation angle of the first worm gear portion 11 or the second worm gearportion 22 provided with the magnets Mp and Mq in response to a changein magnetic flux generated from the magnets Mp and Mq. In the magnets Mpand Mq, a first magnetic pole portion N of a first polarity (N-pole) anda second magnetic pole portion S of a second polarity (S-pole) differentfrom the first polarity are formed adjacent to each other as viewed fromend surfaces (upper surfaces Mpa and Mqa and lower surfaces Mpb and Mqb)of the magnets Mp and Mq in axial directions MpC and MqC. The firstmagnetic pole portion N and the second magnetic pole portion S areformed adjacent to each other in the radial direction with the center ofthe magnets Mp and Mq in the radial direction as a boundary, and thefirst magnetic pole portion N and the second magnetic pole portion S areformed adjacent to each other in the axial directions MpC and MqC withthe center in the axial directions MpC and MqC as a boundary. Theabsolute encoder 2 is provided with magnetic interference reductionmembers 16 and 17 formed of a magnetic material on outer peripheralsurfaces Mpd and Mqd in the radial direction of the magnets Mp and Mq.Hereinafter, the structure of the absolute encoder 2 will be describedin detail.

FIG. 1 is a perspective view schematically illustrating theconfiguration of the absolute encoder 2 according to the embodiment ofthe present invention. FIG. 2 is a perspective view illustrating theconfiguration of the absolute encoder 2 in FIG. 1 with a case 4 removed.FIG. 1 is a see-through illustration looking through the case 4 and asubstrate 5 of the absolute encoder 2, and FIG. 2 is a see-throughillustration looking through the substrate 5 of the absolute encoder 2.

In the present embodiment, for convenience of explanation, the absoluteencoder 2 will be described with reference to an XYZ Cartesiancoordinate system. The X-axis direction corresponds to a horizontalleft-right direction, the Y-axis direction corresponds to a horizontalfront-rear direction, and the Z-axis direction corresponds to a verticalup-down direction. The Y-axis direction and the Z-axis direction areorthogonal to the X-axis direction. In the present description, theX-axis direction is also referred to as the left side or the right side,the Y-axis direction is also referred to as the front side or the rearside, and the Z-axis direction is also referred to as the upper side orthe lower side. The absolute encoder 2 illustrated in FIGS. 1 and 2 isorientated such that the left side in the X-axis direction is the leftside and the right side in the X-axis direction is the right side.Further, the absolute encoder 2 illustrated in FIGS. 1 and 2 isorientated such that the near side in the Y-axis direction is the frontside and the back side in the Y-axis direction is the rear side.Additionally, the absolute encoder 2 illustrated in FIGS. 1 and 2 isorientated such that the upper side in the Z-axis direction is the upperside and the lower side in the Z-axis direction is the lower side. Astate viewed from the upper side in the Z-axis direction is referred toas a plan view, a state viewed from the front side in the Y-axisdirection is referred to as a front view, and a state viewed from theX-axis direction is referred to as a side view. The notation for suchdirections is not intended to limit the usage orientation of theabsolute encoder 2, and the absolute encoder 2 may be used in anyorientation.

As described above, the absolute encoder 2 is an absolute-type encoderidentifying and outputting an amount of rotation of the main shaft la ofa motor 1 over multiple rotations. In the embodiment of the presentinvention, the absolute encoder 2 is provided at an end portion at theupper side in the Z-axis direction of the motor 1. In the embodiment ofthe present invention, the absolute encoder 2 has a substantiallyrectangular shape in plan view, and has a rectangular shape being thinand long in the up-down direction being the extension direction of themain shaft la in front view and side view. That is, the absolute encoder2 has a flat rectangular parallelepiped shape being longer in thehorizontal direction than in the up-down direction.

The absolute encoder 2 includes a hollow and angular tubular case 4accommodating the internal structure. The case 4 has a plurality of(e.g., four) outer wall portions 4 a surrounding at least a portion ofthe main shaft la of the motor 1, a main shaft gear 10, and anintermediate gear 20. Furthermore, a lid portion 4 b is formedintegrally with the four outer wall portions 4 a of the case 4 at upperend portions of the outer wall portions 4 a.

The motor 1 may be a stepper motor or a brushless DC motor, for example.As an example, the motor 1 may be a motor employed as a drive source fordriving an industrial robot via a reduction mechanism such as strainwave gearing. The main shaft la of the motor 1 projects from the case ofthe motor at both sides in the up-down direction. The absolute encoder 2outputs the amount of rotation of the main shaft la of the motor 1 as adigital signal.

The motor 1 has a substantially rectangular shape in plan view, and alsohas a substantially rectangular shape in the up-down direction. That is,the motor 1 has a substantially cuboid shape. In plan view, the fourouter wall portions constituting the outer shape of the motor 1 eachhave a length of 25 mm, for example. In other words, the external shapeof the motor 1 is a 25 mm square in plan view. The absolute encoder 2provided in the motor 1 is, for example, a 25 mm square to match theexternal shape of the motor 1.

In FIGS. 1 and 2 , the substrate 5 is provided covering the inside ofthe absolute encoder 2 together with the case 4. The substrate 5 has asubstantially rectangular shape in plan view, and is a plate-likeprinted wiring substrate being thin in the up-down direction. Aconnector 6 is connected to the substrate 5 and is used for coupling theabsolute encoder 2 to an external device (not illustrated).

FIG. 3 is a perspective view schematically illustrating theconfiguration of the absolute encoder 2 in FIG. 2 with the substrate 5and the connector 6 removed. FIG. 4 is a perspective view schematicallyillustrating the configuration of the absolute encoder 2 in FIG. 3 whenviewed from another angle. FIG. 5 is a perspective view schematicallyillustrating the configuration of the absolute encoder 2 in FIG. 3 withthe motor 1 removed. FIG. 6 is a plan view schematically illustratingthe configuration of the absolute encoder 2 in FIG. 5 .

The absolute encoder 2 includes the main shaft gear 10 including thefirst worm gear portion 11 (first drive gear), the intermediate gear 20including the first worm wheel portion 21 (first driven gear) and thesecond worm gear portion 22 (second drive gear), a secondary shaft gear30 including the second worm wheel portion 31 (second driven gear), amagnet Mp, an angle sensor Sp corresponding to the magnet Mp, a magnetMq, an angle sensor Sq corresponding to the magnet Mq, and amicrocomputer 51.

The main shaft 1 a of the motor 1 is an output shaft of the motor 1, andis an input shaft transmitting rotational force to the absolute encoder2. The main shaft gear 10 is fixed to the main shaft 1 a of the motor 1,and is rotatably supported by a bearing member of the motor 1 integrallywith the main shaft 1 a. The first worm gear portion 11 is provided atthe outer periphery of the main shaft gear 10 so as to rotate accordingto the rotation of the main shaft 1 a of the motor 1. In the main shaftgear 10, the first worm gear portion 11 is provided so that the centralaxis of the first worm gear portion 11 coincides with or substantiallycoincides with the central axis of the main shaft 1 a. The first wormwheel portion 21 is provided at the outer periphery of the intermediategear 20. The first worm wheel portion 21 is provided to mesh with thefirst worm gear portion 11 and rotate according to the rotation of thefirst worm gear portion 11. The axial angle between the first worm wheelportion 21 and the first worm gear portion 11 is set to 90° orapproximately 90°.

Although the outer diameter of the first worm wheel portion 21 is notparticularly limited, in the illustrated example, the outer diameter ofthe first worm wheel portion 21 is set to be smaller than the outerdiameter of the first worm gear portion 11 (see FIG. 8 ), and the outerdiameter of the first worm wheel portion 21 is reduced. As a result, thesize of the absolute encoder 2 in the up-down direction is reduced.

The second worm gear portion 22 is provided at the outer periphery ofthe intermediate gear 20, and rotates according to the rotation of thefirst worm wheel portion 21. In the intermediate gear 20, the secondworm gear portion 22 is provided so that the central axis of the secondworm gear portion 22 coincides with or substantially coincides with thecentral axis of the first worm wheel portion 21. The second worm wheelportion 31 is provided at the outer periphery of the secondary shaftgear 30. The second worm wheel portion 31 is provided to mesh with thesecond worm gear portion 22 and rotate according to the rotation of thesecond worm gear portion 22. The axial angle between the second wormwheel portion 31 and the second worm gear portion 22 is set to 90° orapproximately 90°. The rotation axis line of the second worm wheelportion 31 is parallel or substantially parallel with the rotation axisline of the first worm gear portion 11.

Here, the first worm wheel portion 21 moves toward the first worm gearportion 11 to mesh with the first worm gear portion 11 in a direction.This direction is defined as a first meshing direction (directionindicated by an arrow P1 in FIG. 12 ). Similarly, the second worm gearportion 22 moves toward the second worm wheel portion 31 to mesh withthe second worm wheel portion 31 in a direction. This direction isdefined as a second meshing direction (direction indicated by an arrowP2 in FIG. 12 ). In the present embodiment, the first meshing directionP1 and the second meshing direction P2 are both a direction along ahorizontal plane (XY plane).

The angle sensor Sq detects the rotation angle of the second worm wheelportion 31, that is, the rotation angle of the secondary shaft gear 30.The magnet Mq is fixed to an upper surface of the secondary shaft gear30 such that the central axes of both the magnet Mq and the secondaryshaft gear 30 coincide or substantially coincide. The magnet Mq has twomagnetic poles (N/S) aligned in a direction perpendicular to therotation axis line of the secondary shaft gear 30. Specifically, in themagnet Mq, with the center as a boundary, the first magnetic poleportion N and the second magnetic pole portion S are arrayed in an axialdirection MqC of the magnet Mq in the left half of FIG. 15 . On theother hand, in the right half of FIG. 15 , the first magnetic poleportion N and the second magnetic pole portion S are arrayed in theaxial direction MqC in FIG. 15 of the magnet Mq in an inverted up-downdirection to the left half. That is, in the magnet Mq, the firstmagnetic pole portion N and the second magnetic pole portion S areprovided adjacent to each other in the radial direction with the centerof the magnet Mq in the radial direction as a boundary, and the firstmagnetic pole portion N and the second magnetic pole portion S areprovided adjacent to each other in the axial direction MqC with thecenter in the axial direction MqC as a boundary. The magnetizationdirection of the magnet Mq is the axial direction MqC, that is,magnetization in the plane direction (plane magnetization). To detectthe rotation angle of the secondary shaft gear 30, the angle sensor Sqis provided such that a lower surface of the angle sensor Sq opposes anupper surface of the magnet Mq across a gap in the up-down direction.

As an example, the angle sensor Sq is fixed to the substrate 5, and thesubstrate 5 is supported by substrate pillars 110 disposed at a base 3(described later) of the absolute encoder 2. The angle sensor Sq detectsthe magnetic pole of the magnet Mq, and outputs the detectioninformation to the microcomputer 51. The microcomputer 51 identifies therotation angle of the magnet Mq, that is, the rotation angle of thesecondary shaft gear 30 based on the input detection information relatedto the magnetic pole.

The magnet Mp is fixed to an upper surface of the main shaft gear 10such that the central axes of both the magnet Mp and the main shaft gear10 coincide or substantially coincide. The magnet Mp has two magneticpoles (N/S) aligned in a direction perpendicular to the rotation axisline of the main shaft gear 10. Specifically, in the magnet Mp, with thecenter as a boundary, the first magnetic pole portion N and the secondmagnetic pole portion S are arrayed in the axial direction MpC of themagnet Mp in the left half of FIG. 9 . On the other hand, in the righthalf of FIG. 9 , the first magnetic pole portion N and the secondmagnetic pole portion S are arrayed in the axial direction MpC in FIG. 9of the magnet Mp in an inverted up-down direction to the left half. Thatis, in the magnet Mp, the first magnetic pole portion N and the secondmagnetic pole portion S are provided adjacent to each other in theradial direction with the center of the magnet Mp in the radialdirection as a boundary, and the first magnetic pole portion N and thesecond magnetic pole portion S are provided adjacent to each other inthe axial direction MpC with the center in the axial direction MpC as aboundary. The magnetization direction of the magnet Mp is the axialdirection MpC, that is, magnetization in the plane direction (planemagnetization). To detect the rotation angle of the main shaft gear 10,the angle sensor Sp is provided such that a lower surface of the anglesensor Sp opposes an upper surface of the magnet Mp across a gap in theup-down direction.

As an example, the angle sensor Sq is fixed to a surface of thesubstrate 5, and the angle sensor Sp is fixed to the substrate 5 at thesame surface as the surface. The angle sensor Sp detects the magneticpole of the magnet Mp, and outputs the detection information to themicrocomputer 51. The microcomputer 51 identifies the rotation angle ofthe main shaft gear 10, that is, the rotation angle of the main shaft 1a by identifying the rotation angle of the magnet Mp based on the inputdetection information related to the magnetic pole. The resolution ofthe rotation angle of the main shaft 1 a corresponds to the resolutionof the angle sensor Sp. As described later, the microcomputer 51identifies, and outputs, the amount of rotation of the main shaft 1 abased on the identified rotation angle of the secondary shaft gear 30and the identified rotation angle of the main shaft 1 a. As an example,the microcomputer 51 may output the amount of rotation of the main shaft1 a of the motor 1 as a digital signal.

The absolute encoder 2 configured as described above can identify therotation speed of the main shaft 1 a according to the rotation angle ofthe secondary shaft gear 30 identified based on the detectioninformation of the angle sensor Sq, and can identify the rotation angleof the main shaft 1 a based on the detection information of the anglesensor Sp. Then, the microcomputer 51 identifies the amount of rotationof the main shaft 1 a over multiple rotations based on the identifiedrotation speed of the main shaft 1 a and the rotation angle of the mainshaft 1 a.

The number of threads of the first worm gear portion 11 of the mainshaft gear 10 provided at the main shaft 1 a is, for example, five, andthe number of teeth of the first worm wheel portion 21 is, for example,20. In other words, the first worm gear portion 11 and the first wormwheel portion 21 constitute a first transmission mechanism R1 having areduction ratio of 20/5=4 (see FIG. 6 ). When the first worm gearportion 11 rotates four times, the first worm wheel portion 21 rotatesone time. Because the first worm wheel portion 21 and the second wormgear portion 22 are provided coaxially to constitute the intermediategear 20 and rotate together, when the first worm gear portion 11 rotatesfour times, i.e., when the main shaft 1 a and the main shaft gear 10rotate four times, the intermediate gear 20 rotates one time and thesecond worm gear portion 22 rotates one time.

The number of threads of the second worm gear portion 22 is, forexample, two, and the number of teeth of the second worm wheel portion31 is, for example, 25. That is, the second worm gear portion 22 and thesecond worm wheel portion 31 constitute a second transmission mechanismR2 having a reduction ratio of 25/2=12.5 (see FIG. 6 ). When the secondworm gear portion 22 rotates 12.5 times, the second worm wheel portion31 rotates one time. Because the secondary shaft gear 30 formed with thesecond worm wheel portion 31 is configured to rotate integrally with themagnet Mq and a magnet holder 35 as will be described later, the magnetMq rotates one time when the second worm gear portion 22 constitutingthe intermediate gear 20 rotates 12.5 times. As described above, whenthe main shaft 1 a rotates 50 times, the intermediate gear 20 rotates12.5 times, and the secondary shaft gear 30 and the magnet Mq rotate onetime. In other words, the rotation speed of the main shaft 1 a over 50rotations can be identified by the detection information related to therotation angle of the secondary shaft gear 30 of the angle sensor Sq.

Hereinafter, the configuration of the absolute encoder 2 will bedescribed in further detail.

As described above (see FIGS. 1 to 6 ), the absolute encoder 2 includesthe base 3, the case 4, the substrate 5, and the connector 6. Theabsolute encoder 2 includes the main shaft gear 10, the intermediategear 20, the secondary shaft gear 30, and the biasing mechanism 40. Theabsolute encoder 2 includes the magnets Mp and Mq and the angle sensorsSp and Sq, and includes the microcomputer 51 for controlling a driveunit or detection unit of the absolute encoder 2.

The base 3 is a base rotatably holding rotating bodies such as the mainshaft gear 10, the intermediate gear 20, and the secondary shaft gear30, and fixing members such as the substrate 5 and the biasing mechanism40. As illustrated in FIGS. 3 to 6 , FIGS. 10 to 14 , and others, thebase 3 includes a base portion 101 and various support portions to bedescribed below for supporting each member of the absolute encoder 2provided at the base portion 101. The case 4 is fixed to the base 3 byhook portions at three locations, for example, and by a screw at onelocation. Further, the substrate 5 is configured to be fixed to the base3 by screws at three locations, for example.

The base portion 101 is a plate-like portion having a pair of surfacesfacing the up-down direction of the absolute encoder 2, and extends inthe horizontal direction (X-axis direction and Y-axis direction). Asillustrated in FIG. 7 , recess parts 103 engageable with hook portions 4c formed at the case 4 are formed in a lower surface 102 at a lower sideof the base portion 101. For example, three of the recess parts 103 areformed, as described above.

The substrate pillars 110 and substrate positioning pins 120 beingportions for supporting the substrate 5 are provided at an upper surface104 being a surface at the upper side of the base portion 101. The base3 includes, for example, three of the substrate pillars 110 and two ofthe substrate positioning pins 120.

As illustrated in FIG. 5 and others, the substrate pillar 110 is aportion projecting upward from the upper surface 104 of the base portion101, and is, for example, a columnar or substantially columnar portion.A screw hole 112 extending downward is formed in an upper-side endsurface (an upper end surface 111) of the substrate pillar 110. Theupper end surfaces 111 of the substrate pillars 110 are formed so as toextend on the same horizontal plane or extend along the same horizontalplane. In the absolute encoder 2, a lower surface 5 a of the substrate 5is in contact with the upper end surfaces 111 of the substrate pillars110, and the substrate 5 is fixed to the substrate pillars 110 by screws8 a screwed into the screw holes 112. Note that, as described below, oneof the substrate pillars 110 is integrated with a support projection 45constituting the one substrate positioning pin 120 and the biasingmechanism 40 described below.

As illustrated in FIG. 5 and others, the substrate positioning pin 120is a portion projecting upward from the upper surface 104 of the baseportion 101, and is, for example, a columnar or a substantially columnarportion. An upper end portion (a tip end portion 121) of the substratepositioning pin 120 is narrower than a portion (a base portion 122)being lower than the tip end portion 121, and a stepped surface 123 isformed between the tip end portion 121 and the base portion 122. The tipend portion 121 of the substrate positioning pin 120 can be insertedinto a positioning hole 5 b formed in the substrate 5 as illustrated inFIG. 20 to be described below. The substrate 5 is positioned relative tothe base 3 by inserting the tip end portion 121 of the substratepositioning pin 120 into the positioning hole 5 b of the substrate 5.

As illustrated in FIG. 5 and others, the base 3 includes a supportprojection 131 provided at the upper surface 104 of the base portion 101and being a portion projecting upward. A screw hole 131 a extending inthe horizontal direction (left-right direction in the figure) is formedin the support projection 131. A screw 8 b is screwed into the screwhole 131 a of the support projection 131 to fix the case 4 to the base 3via the screw 8 b.

The base 3 includes support projections 132, 141 and 142 provided at theupper surface 104 of the base portion 101 and being portions projectingupward (see FIGS. 3 to 6 , etc.). The support projection 132 is aportion supporting a leaf spring 9 configured to push the intermediategear 20 in the central axial direction of the intermediate gear 20, asdescribed below. The support projections 141 and 142 are portions forrotatably supporting the intermediate gear 20, as described below.Furthermore, the base 3 includes a bearing holder portion 134 supportinga bearing 135 rotatably holding the secondary shaft gear 30, asdescribed below (see FIG. 15 ). Additionally, the support projection 45is formed at the upper surface 104 of the base portion 101 of the base3. As described below, the support projection 45 is a portionconstituting the biasing mechanism 40 biasing the second worm gearportion 22 in the direction of the second worm wheel portion 31, and isa portion supporting a biasing spring 41.

Next, each component of the absolute encoder 2 supported by the base 3will be described in detail.

Main Shaft Gear

FIG. 8 is a cross-sectional view schematically illustrating theconfiguration of the absolute encoder 2 in FIG. 1 with the motor 1removed, sectioned along a plane through the central axis of the mainshaft gear 10 and orthogonal to the central axis of the intermediategear 20. FIG. 9 is an enlarged cross-sectional view schematicallyillustrating portions around the magnet Mp and the magnetic interferencereduction member 16 in the configuration of the absolute encoder 2 inFIG. 8 . FIG. 10 is an exploded longitudinal cross-sectional viewschematically illustrating configurations of the magnet Mp, the mainshaft gear 10, a main shaft adapter 12, and the main shaft 1 a of themotor 1 in the configuration of the absolute encoder 2 in FIG. 8 .

As illustrated in FIGS. 8, 9, and 10 , the main shaft gear 10 is atubular member provided coaxially or substantially coaxially with themain shaft 1 a of the motor 1 and the main shaft adapter 12. The mainshaft gear 10 includes a tubular portion 13 having a tubular shape andthe first worm gear portion 11 provided at the outer side in the radialdirection of the tubular portion 13. The first worm gear portion 11 is agear portion of the main shaft gear 10. As illustrated in FIG. 10 , apress-fitting section 1 b in the form of a cylindrical surface andforming a space at the inner peripheral side is formed in the upper endof the main shaft 1 a of the motor 1. The press-fitting section 1 b hasa shape allowing the main shaft adapter 12 to be press-fitted and fixed.Further, the tubular portion 13 of the main shaft gear 10 is formed witha press-fitting section 14 in the form of a cylindrical surface andforming a space at an inner side. The press-fitting section 14 has ashape allowing the main shaft adapter 12 to be press-fitted and fixed.

As illustrated in FIGS. 8, 9, and 10 , a magnet holding portion 15configured to hold the magnet Mp is formed in the tubular portion 13 ofthe main shaft gear 10. The magnet holding portion 15 is a portionforming a recess part corresponding to the shape of the magnet Mp andbeing recessed downward from an upper end surface 13 a of the tubularportion 13. The magnet holding portion 15 can accommodate the magnet Mp.The magnet holding portion 15 has an inner peripheral surface 15 a inthe form of a cylindrical surface communicating with the press-fittingsection 14 and having a larger diameter than the press-fitting section14, and an annular bottom surface 15 b connecting the inner peripheralsurface 15 a and the press-fitting section 14.

The annular magnetic interference reduction member 16 formed surroundingthe entire periphery of the outer peripheral surface Mpd of the magnetMp is provided on the outer peripheral surface Mpd in the radialdirection of the magnet Mp (hereinafter, simply referred to as “outerperipheral surface”). An inner peripheral surface 16 a of the magneticinterference reduction member 16 is formed corresponding to the outerdiameter of the magnet Mp and the shape of the outer peripheral surfaceMpd to be in contact with the outer peripheral surface Mpd of the magnetMp. The inner peripheral surface 16 a of the magnetic interferencereduction member 16 and the outer peripheral surface Mpd of the magnetMp are preferably in contact with each other so as not to form a gap.

The inner peripheral surface 15 a of the magnet holding portion 15 isformed to be in contact with an outer peripheral surface Mpd of themagnet Mp accommodated in the magnet holding portion 15. In the absoluteencoder 2, an upper end surface 12 a of the main shaft adapter 12 ispositioned above the bottom surface 15 b of the magnet holding portion15. In the absolute encoder 2, a bottom surface Mpb of the magnet Mp isin contact with the upper end surface 12 a of the main shaft adapter 12,and is not in contact with the bottom surface 15 b of the magnet holdingportion 15 of the main shaft gear 10. Thus, the magnet Mp is positionedin the up-down direction by the upper end surface 12 a of the main shaftadapter 12 and positioned in the horizontal direction by the innerperipheral surface 15 a of the magnet holding portion 15. The lowersurface Mpb of the magnet Mp positioned in this manner is bonded andfixed to the upper end surface 12 a of the main shaft adapter 12.

As described above, the magnet Mp is fixed to the main shaft adapter 12,and the magnet Mp, the main shaft gear 10, and the main shaft adapter 12rotate integrally with the main shaft 1 a of the motor 1. The magnet Mp,the main shaft gear 10, and the main shaft adapter 12 are configured torotate about the same axis line as the main shaft 1 a of the motor 1.

The first worm gear portion 11 is constituted by a teeth portion formedinto a helical shape, and is formed to mesh with the first worm wheelportion 21 of the intermediate gear 20. The first worm gear portion 11is formed of, for example, polyacetal resin. The first worm gear portion11 is an example of a first drive gear.

As illustrated in FIG. 10 , the magnet Mp is a disk-shaped orsubstantially disk-shaped permanent magnet inserted into the magnetholding portion 15 of the main shaft gear 10, and has the upper surfaceMpa and the lower surface Mpb opposing each other. In the absoluteencoder 2, the position (position in the up-down direction) of themagnet Mp in a direction of a central axis GmC of the main shaft gear 10is defined by the upper end surface 12 a of the main shaft adapter 12,as described above, so that the upper surface MPa of the magnet Mpopposes the surface of the angle sensor Sp across a certain distance inthe up-down direction.

A central axis MpC of the magnet Mp (axis representing the center of themagnet Mp or axis passing through the center of a magnetic poleboundary) coincides or substantially coincides with the central axis GmCof the main shaft gear 10, a central axis SaC of the main shaft adapter12, and a central axis MoC of the main shaft 1 a of the motor 1. Whenthese central axes are made to coincide or substantially coincide witheach other, the angle sensor Sp can detect the rotation angle or theamount of rotation of the magnet Mp with higher accuracy.

Note that in the embodiment of the present invention, the two magneticpoles (N/S) of the magnet Mp are preferably formed adjacent in ahorizontal plane (XY plane) perpendicular to the central axis MpC of themagnet Mp. With this configuration, the detection accuracy of therotation angle or amount of rotation by the angle sensor Sp can befurther improved. Note that the magnet Mp is formed from a magneticmaterial such as a ferritic material, an Nd (neodymium)—Fe (iron)—B(boron) material. The magnet Mp may be, for example, a rubber magnet ora bonded magnet including a resin binder.

Next, the action of the magnetic interference reduction member 17 of theabsolute encoder 2 will be described.

In the absolute encoder 2, as described above, in the magnet Mq, thefirst magnetic pole portion N and the second magnetic pole portion S areprovided adjacent to each other in the radial direction with the centerof the magnet Mq in the radial direction as a boundary, and the firstmagnetic pole portion N and the second magnetic pole portion S areprovided adjacent to each other in the axial direction MqC with thecenter in the axial direction MqC as a boundary, and the magnet Mq ismagnetized in the plane direction. Therefore, in the magnet Mq, amagnetic flux other than the magnetic flux necessary for detecting therotation angle by the angle sensor Sq, that is, a leakage flux, affectsthe surroundings of another angle sensor or the like.

Here, in the absolute encoder 2, the annular magnetic interferencereduction member 17 formed of a magnetic member is provided on the outerperipheral surface Mqd of the magnet Mq in a manner surrounding theentire periphery of the outer peripheral surface Mqd. An innerperipheral surface 17 a of the magnetic interference reduction member 17is provided to be in contact with the outer peripheral surface Mqd ofthe magnet Mq. Because the magnetic interference reduction member 17 isprovided on the outer peripheral surface Mqd, the leakage flux from themagnet Mq toward the radially outer peripheral side is suppressed in theabsolute encoder 2. That is, in the absolute encoder 2, it is possibleto reduce magnetic interference on surroundings by the magnet Mq whilemaintaining the magnetic flux necessary for detecting the rotation angleby the angle sensor Sq. In particular, in the absolute encoder 2, theinfluence of the leakage flux from the magnet Mq fixed to the uppersurface of the secondary shaft gear 30 such that both central axescoincide or substantially coincide with each other, on the angle sensorSp used for detecting the rotation angle of the main shaft gear 10, ispreferably reduced. Therefore, in the absolute encoder 2, it isparticularly effective to provide the magnetic interference reductionmember 17 capable of reducing the influence of the leakage fluxgenerated by the magnet Mq.

Intermediate Gear

FIG. 11 is a cross-sectional view schematically illustrating theconfiguration of the absolute encoder 2 in FIG. 6 , sectioned along aplane through the central axis of the intermediate gear 20 and parallelto a horizontal plane (XY plane). FIG. 12 is an enlarged perspectiveview of the absolute encoder 2 sectioned as illustrated in FIG. 11 , asviewed from above at a secondary shaft-side end portion 23 b side of anintermediate gear shaft 23. FIG. 13 is a partial cross-sectional viewschematically illustrating the configuration of the absolute encoder 2in FIG. 6 , sectioned along a plane through the central axis of theintermediate gear 20 and orthogonal to the horizontal plane (XY plane).

As illustrated in FIGS. 4 to 6 and FIGS. 11 to 13 , the intermediategear 20 is rotatably supported by the intermediate gear shaft 23 at theupper side of the base portion 101 of the base 3. The intermediate gearshaft 23 extends parallel to the horizontal plane. Additionally, theintermediate gear shaft 23 is not parallel to the left-right direction(X-axis direction) and the front-rear direction (Y-axis direction) in aplan view. In other words, the intermediate gear shaft 23 is inclinedwith respect to the left-right direction and the front-rear direction.The intermediate gear shaft 23 being inclined with respect to theleft-right direction and the front-rear direction means that theintermediate gear shaft 23 extends obliquely with respect to outerperipheral surfaces 105 to 108 (right-side outer peripheral surface 105,rear-side outer peripheral surface 106, left-side outer peripheralsurface 107, and front-side outer peripheral surface 108) of the baseportion 101 of the base 3 (see FIG. 11 ). In the absolute encoder 2, theintermediate gear shaft 23 is supported by the base portion 101 of thebase 3 by the support projection 141 located at the main shaft gear 10side and the support projection 142 located at the secondary shaft gear30 side.

As illustrated in FIG. 11 , the outer peripheral surface of the base 3is constituted by the right-side outer peripheral surface 105 and theleft-side outer peripheral surface 107 parallel to the YZ plane, and therear-side outer peripheral surface 106 and the front-side outerperipheral surface 108 parallel to the XZ plane and extending betweenthe right-side outer peripheral surface 105 and the left-side outerperipheral surface 107. The right-side outer peripheral surface 105 is aside surface provided at the right side (right side in the X-axisdirection) of the base 3. The left-side outer peripheral surface 107 isa side surface provided at the left side (left side in the X-axisdirection) of the base 3. The rear-side outer peripheral surface 106 isa side surface provided at the rear side (rear side in the Y-axisdirection) of the base 3. The front-side outer peripheral surface 108 isa side surface provided at the front side (front side in the Y-axisdirection) of the base 3.

As illustrated in FIGS. 3 to 6 , the dimensions of the absolute encoder2 in a plan view are aligned with the dimensions of the motor 1 being 25mm square. Thus, because the intermediate gear 20 disposed parallel tothe upper surface 104 of the base 3 is provided to extend obliquely withrespect to the outer peripheral surfaces 105 to 108 of the base 3, thedimensions of the absolute encoder 2 in the horizontal direction can bereduced. Note that the horizontal direction is a direction equal to adirection orthogonal to the central axis of the main shaft 1 a of themotor 1, and is a direction equal to the direction parallel to the XYplane.

As illustrated in FIGS. 5 and 6 , and FIGS. 11 to 14 , the intermediategear 20 is a tubular member formed rotatably about the intermediate gearshaft 23, and includes the first worm wheel portion 21, the second wormgear portion 22, a tubular portion 24, a main shaft-side sliding portion25, and a secondary shaft-side sliding portion 26. The tubular portion24 is a member extending in a tubular shape and has an inner peripheralsurface 24 b forming a through-hole 24 a. The intermediate gear shaft 23can be inserted into the through-hole 24 a. The through-hole 24 a is aspace surrounded by the inner peripheral surface 24 b of the tubularportion 24. The inner peripheral surface 24 b is formed in a slidablemanner at the outer peripheral surface of the intermediate gear shaft23, being inserted through the through-hole 24 a. Further, theintermediate gear 20 is supported by the intermediate gear shaft 23rotatably about the intermediate gear shaft 23. The intermediate gear 20is an integrally formed member made of metal, resin, or the like. Inthis embodiment, the intermediate gear 20 is formed of polyacetal resinas an example.

As illustrated in FIGS. 5 to 8 , the first worm wheel portion 21 is agear meshing with the first worm gear portion 11 of the main shaft gear10. The first worm wheel portion 21 is an example of a first drivengear. The first worm wheel portion 21 is provided at one end portionside of the tubular portion 24 of the intermediate gear 20, and isconstituted by a plurality of teeth provided at a cylindrical surfaceformed at one end portion side of the tubular portion 24 of theintermediate gear 20, for example. In the absolute encoder 2, theintermediate gear 20 is provided such that the first worm wheel portion21 is located near the center of the base portion 101 of the base 3.Accordingly, the one end portion of the tubular portion 24 provided inthe vicinity of the first worm wheel portion 21 is an end portion of theintermediate gear 20 at the main shaft gear 10 side.

As illustrated in FIG. 8 , the outer diameter of the first worm wheelportion 21 is smaller than the outer diameter of the first worm gearportion 11. The central axis of the first worm wheel portion 21 iscoaxial or substantially coaxial with the central axis of the innerperipheral surface 24 b of the tubular portion 24. In the absoluteencoder 2, because the central axis of the first worm wheel portion 21is parallel with the upper surface 104 of the base portion 101 of thebase 3, the outer diameter of the first worm wheel portion 21 isdecreased, and thus the size of the absolute encoder 2 in the up-downdirection (height direction) can be reduced.

As illustrated in FIGS. 5, 6, 11 to 15 , and others, the second wormgear portion 22 is formed by a teeth portion formed into a helicalshape, and is disposed coaxially or substantially coaxially with thefirst worm wheel portion 21. The second worm gear portion 22 is anexample of a second drive gear. Specifically, the second worm gearportion 22 is provided at the other end portion side of the tubularportion 24, and is constituted by the teeth portion formed into ahelical shape provided at a cylindrical surface formed at the other endportion side of the tubular portion 24, for example. The other endportion side of the tubular portion 24 is a side of the end portion ofthe intermediate gear 20 at the secondary shaft gear 30 side.Additionally, the central axis of the second worm gear portion 22 iscoaxial or substantially coaxial with the central axis of the innerperipheral surface 24 b of the tubular portion 24. When the second wormgear portion 22 meshes with the second worm wheel portion 31 provided atthe secondary shaft gear 30, the rotational force of the intermediategear 20 is transmitted to the secondary shaft gear 30.

As described above, the axial angle between the first worm gear portion11 and the first worm wheel portion 21 is 90° or substantially 90°, andthe central axis of the first worm gear portion 11 and the central axisof the first worm wheel portion 21 are orthogonal or substantiallyorthogonal to each other when viewed from a direction perpendicular tothe central axis of the first worm gear portion 11 and perpendicular tothe central axis of the first worm wheel portion 21. Similarly, theaxial angle between the second worm gear portion 22 and the second wormwheel portion 31 is 90° or substantially 90°, and the central axis ofthe second worm gear portion 22 and the central axis of the second wormwheel portion 31 are orthogonal or substantially orthogonal to eachother when viewed from a direction perpendicular to the central axis ofthe second worm gear portion 22 and perpendicular to the central axis ofthe second worm wheel portion 31.

As illustrated in FIG. 15 , the outer diameter of the second worm gearportion 22 is set to the smallest possible value to achieveminiaturization of the absolute encoder 2 in the up-down direction(height direction).

As illustrated in FIGS. 6 and 11 to 13 , the main shaft-side slidingportion 25 of the intermediate gear 20 is provided at an end of theintermediate gear 20, that is, at an end of the intermediate gear 20 atthe main shaft gear 10 side. Specifically, the main shaft-side slidingportion 25 is an end surface of one end of the tubular portion 24, andis an annular surface facing the central axial direction of theintermediate gear 20 formed at one end of the tubular portion 24. In theabsolute encoder 2, the main shaft-side sliding portion 25 of theintermediate gear 20 is in contact with a first end 9 a of the leafspring 9 to be described later.

The leaf spring 9 is an example of an elastic member and is made ofmetal, for example. The leaf spring 9 is a member for pushing theintermediate gear 20 in the central axial direction of the intermediategear shaft 23 in the absolute encoder 2, and as illustrated in FIGS. 4to 6 and FIG. 13 , a second end 9 b of the leaf spring 9 is fixed to thesupport projection 132 of the base 3 by a screw 8 c to be supported bythe base 3. The first end 9 a of the leaf spring 9 is formed to be incontact with the main shaft-side sliding portion 25 of the intermediategear 20. Specifically, as illustrated in FIGS. 4 and 13 , the first end9 a of the leaf spring 9 is constituted by two branched portions dividedinto two prongs. A gap greater than the diameter of the intermediategear shaft 23 is formed between the two branched portions constitutingthe first end 9 a of the leaf spring 9. In this way, in the absoluteencoder 2, the two branched portions of the first end 9 a of the leafspring 9 pass through the intermediate gear shaft 23 and are in contactwith the main shaft-side sliding portion 25 of the intermediate gear 20.

As illustrated in FIGS. 4, 6, 11, and 13 , the leaf spring 9 is fixed tothe support projection 132 of the base 3 at the second end 9 b such thatthe first end 9 a is in contact with the main shaft-side sliding portion25 of the intermediate gear 20 in a state of the leaf spring 9 beingdeflected in the absolute encoder 2. Thus, an elastic force is generatedin the leaf spring 9, and the main shaft-side sliding portion 25 of theintermediate gear 20 is pressed by the first end 9 a of the leaf spring9. This pressing force of the leaf spring 9 biases the intermediate gear20 in a direction from the support projection 141 at the main shaft gear10 side toward the support projection 142 at the secondary shaft gear 30side along the intermediate gear shaft 23. When the intermediate gear 20rotates in this state, the main shaft-side sliding portion 25 of theintermediate gear 20 rotates while in contact with the first end 9 a ofthe leaf spring 9.

As illustrated in FIGS. 4 and 6 , and FIGS. 11 to 14 , the secondaryshaft-side sliding portion 26 of the intermediate gear 20 is provided atthe other end of the intermediate gear 20, that is, at an end of theintermediate gear 20 at the secondary shaft gear 30 side. Specifically,the secondary shaft-side sliding portion 26 is an end surface of theother end of the tubular portion 24, and is an annular surface facingthe central axial direction of the intermediate gear 20 formed at theother end of the tubular portion 24, and opposes the main shaft-sidesliding portion 25 in the central axial direction of the intermediategear 20.

In the absolute encoder 2, the secondary shaft-side sliding portion 26of the intermediate gear 20 is in contact with the support projection142, and the support projection 142 defines the position of theintermediate gear 20 in the central axial direction of the intermediategear shaft 23. As described above, because the intermediate gear 20 ispressed by the leaf spring 9 in a direction from the support projection141 at the main shaft gear 10 side toward the support projection 142 atthe secondary shaft gear 30 side, the secondary shaft-side slidingportion 26 of the intermediate gear 20 is also pressed in the samedirection to be in contact with the support projection 142. In thismanner, the pressing force of the leaf spring 9 is transmitted from thesecondary shaft gear 30 to the support projection 142, and theintermediate gear 20 is stably supported in the direction from thesupport projection 141 toward the support projection 142. When theintermediate gear 20 rotates, the secondary shaft-side sliding portion26 of the intermediate gear 20 rotates while being in contact with thesupport projection 142.

The support projection 141 and the support projection 142 are an exampleof a first shaft support portion and a second shaft support portionrotatably holding the intermediate gear 20 via the intermediate gearshaft 23, respectively. As illustrated in FIGS. 5 and 6 , and FIGS. 11to 13 , the support projection 141 and the support projection 142 form apair and are substantially rectangular portions projecting upward fromthe base portion 101 of the base 3, for example. The support projection141 is formed near the main shaft gear 10, near the left side of thebase 3 and near the center of the base 3 in the front-rear direction ina plan view (see FIGS. 6 and 10 ). Additionally, the support projection142 is formed near the secondary shaft gear 30, at the right side andthe front side of the base 3 in a plan view.

As illustrated in FIG. 6 and FIGS. 11 to 13 , the support projection 141and the support projection 142 function as a support member slidablysupporting the intermediate gear shaft 23 along a horizontal plane, thatis, function as a support member slidably supporting the intermediategear 20 along a horizontal plane. The intermediate gear shaft 23 is acolumnar rod-like member, and includes a main shaft-side end portion 23a as a first end portion and a secondary shaft-side end portion 23 b asa second end portion. The main shaft-side end portion 23 a is an endportion of the intermediate gear shaft 23 located at the main shaft gear10 side in the absolute encoder 2, and the secondary shaft-side endportion 23 b is an end portion of the intermediate gear shaft 23 locatedat the secondary shaft gear 30 side in the absolute encoder 2.

By the biasing mechanism 40 described below, the first worm wheelportion 21 provided at the main shaft-side end portion 23 a side of theintermediate gear shaft 23 can be moved in the first meshing direction(direction indicated by the arrow P1 in FIG. 12 ), but cannot be movedin the extension direction of the intermediate gear shaft 23 (centralaxial direction of the intermediate gear shaft 23) and a directionorthogonal to the first meshing direction P1 (up-down direction). Asdescribed above, the first worm wheel portion 21 moves toward the firstworm gear portion 11 to mesh with the first worm gear portion 11 in adirection. This direction is the first meshing direction.

As illustrated in FIGS. 11 to 14 , a through-hole 143 is formed in thesupport projection 141. The main shaft-side end portion 23 a of theintermediate gear shaft 23 is inserted into the through-hole 143. In across section orthogonal to the extension direction of the through-hole143, the through-hole 143 has a round hole shape. The round hole shapeis a shape having a perfect circle or a substantially perfect circleprofile.

The absolute encoder 2 further includes a snap ring 144 as a fixingportion formed engageable with the main shaft-side end portion 23 a ofthe intermediate gear shaft 23. The snap ring 144 is a member forming aportion in the main shaft-side end portion 23 a of the intermediate gearshaft 23 that cannot pass through the through-hole 143 of the supportprojection 141, and is a member partially increasing the outer diameterof the main shaft-side end portion 23 a of the intermediate gear shaft23. As illustrated in FIGS. 12 and 13 , the snap ring 144 is an annularmember such as an E-ring locking into a groove 23 c formed in theintermediate gear shaft 23. In the absolute encoder 2, the groove 23 cis formed in the main shaft-side end portion 23 a of the intermediategear shaft 23 such that the snap ring 144 is positioned at the sideopposite to the secondary shaft-side end portion 23 b side relative tothe support projection 141.

That is, the snap ring 144 is disposed in contact with an outer sidesurface 141 a of the support projection 141. The outer side surface 141a is a surface of the support projection 141 facing a side opposite tothe support projection 142 side. With this configuration, the movementof the intermediate gear shaft 23 is restricted in a direction from themain shaft-side end portion 23 a toward the secondary shaft-side endportion 23 b due to contact with the outer side surface 141 a of thesupport projection 141 of the snap ring 144.

By the biasing mechanism 40 to be described below, the second worm gearportion 22 provided at the secondary shaft-side end portion 23 b side ofthe intermediate gear shaft 23 can be moved in the second meshingdirection (direction indicated by the arrow P2 in FIG. 12 ), and theintermediate gear shaft 23 cannot be moved in the extending direction ofthe intermediate gear shaft 23 (central axial direction of theintermediate gear shaft 23) and a direction orthogonal to the secondmeshing direction P2 (Z-axis direction). As described above, the secondworm gear portion 22 moves toward the second worm wheel portion 31 tomesh with the second worm wheel portion 31 in a direction. Thisdirection is the second meshing direction.

A through-hole 145 is formed in the support projection 142. Thesecondary shaft-side end portion 23 b of the intermediate gear shaft 23is inserted into the through-hole 145. In a cross section orthogonal tothe extension direction of the through-hole 145, the through-hole 145has a long hole shape. The long hole shape of the through-hole 145 has amajor axis and a minor axis orthogonal to the major axis. The majoraxis-side width is greater than the minor axis-side width. The majoraxis-side width of the long hole shape of the through-hole 145 in thesupport projection 142 at the secondary shaft gear 30 side is greaterthan the diameter of the outer peripheral surface of the intermediategear shaft 23. Additionally, the minor axis-side width of thethrough-hole 145 is identical or substantially identical to the diameterof the outer peripheral surface of the intermediate gear shaft 23. Inthe absolute encoder 2, the major axis direction of the through-hole 145in the support projection 142 is parallel or substantially parallel withthe horizontal plane. As will be described later, the biasing spring 41engages with the intermediate gear shaft 23. In the intermediate gearshaft 23, the secondary shaft-side end portion 23 b of the intermediategear shaft 23 is inserted into the through-hole 145 of the supportprojection 142. The biasing spring 41 is configured to bias thesecondary shaft-side end portion 23 b of the intermediate gear shaft 23in the second meshing direction P2.

In this way, by the biasing mechanism 40, the support projection 141,and the support projection 142 to be described below, the intermediategear shaft 23 is configured such that the secondary shaft-side endportion 23 b can move parallel or substantially parallel with thehorizontal direction with the main shaft-side end portion 23 a as afulcrum (center of oscillation), and the second worm gear portion 22 atthe secondary shaft-side end portion 23 b can move parallel orsubstantially parallel with the horizontal direction over a larger widththan the first worm wheel portion 21 at the main shaft-side end portion23 a side. With this configuration, the intermediate gear shaft 23, thatis, the intermediate gear 20 biased by the biasing mechanism 40 andsupported by the support projection 141 and the support projection 142can oscillate along a horizontal plane (XY plane).

In such a configuration, the amount of movement (amount of oscillation)of the intermediate gear shaft 23 is determined by the depth of thethrough-hole 143 formed in the support projection 141, that is, thethickness of the support projection 141 in the central axial directionof the intermediate gear shaft 23, the clearance between thethrough-hole 143 and the intermediate gear shaft 23, and the majoraxis-side width of the through-hole 145. However, when the clearancebetween the through-hole 143 and the intermediate gear shaft 23 islarge, the intermediate gear shaft 23 is subject to more backlash andbecomes misaligned. Therefore, this clearance is preferably kept small.In order to solve this, forming the support projection 141 of a thinplate or the like to reduce the thickness of the support projection 141,that is, to make the through-hole 143 shallower makes it possible toensure the amount of movement of the intermediate gear shaft 23 whilethe clearance between the through-hole 143 and the intermediate gearshaft 23 is kept small. Note that the amount of movement of theintermediate gear shaft 23 can be defined by the major axis-side widthof the through-hole 145 by setting the amount of movement of theintermediate gear shaft 23 based on the thickness of the supportprojection 141 larger than the amount of movement of the intermediategear shaft 23 based on the major axis-side width of the through-hole145.

Secondary Shaft Gear

FIG. 15 is a partial cross-sectional view schematically illustrating theconfiguration of the absolute encoder 2 in FIG. 2 , sectioned along aplane through the central axis of the secondary shaft gear 30 andorthogonal to the central axis of the intermediate gear 20. FIG. 16 isan exploded perspective view schematically illustrating theconfiguration of the absolute encoder 2 in FIG. 15 with the magnet Mq,the magnet holder 35, the secondary shaft gear 30, and the bearing 135disassembled.

As illustrated in FIGS. 15 and 16 , the secondary shaft gear 30 is acylindrical member. In the secondary shaft gear 30, a shaft portion 35 bof the magnet holder 35 is press-fitted and fixed to the magnet holder35. The secondary shaft gear 30 includes the second worm wheel portion31 and a through-hole 32. The secondary shaft gear 30 is an integrallyformed member made of metal or resin. In this embodiment, the secondaryshaft gear 30 is formed of a polyacetal resin as an example.

The second worm wheel portion 31 is a gear meshing with the second wormgear portion 22 of the intermediate gear 20. The second worm wheelportion 31 is an example of a second driven gear. The second worm wheelportion 31 is composed of, for example, a plurality of teeth provided atthe outer peripheral portion of the upper-side cylindrical portion ofthe secondary shaft gear 30. When the intermediate gear 20 rotates, therotational force of the intermediate gear 20 is transmitted to thesecondary shaft gear 30 via the second worm gear portion 22 and thesecond worm wheel portion 31 of the intermediate gear 20.

As illustrated in FIGS. 15 and 16 , the through-hole 32 is a holeextending along the central axis of the cylindrical secondary shaft gear30. The shaft portion 35 b of the magnet holder 35 is press-fitted intothe through-hole 32, and the secondary shaft gear 30 is configured torotate integrally with the magnet holder 35.

The magnet holder 35 includes a magnet holding portion 35 a and theshaft portion 35 b. The magnet holder 35 is an integrally formed membermade of metal or resin. In this embodiment, the magnet holder 35 isformed of non-magnetic stainless steel as an example. The outer rings oftwo of the bearings 135 are press-fitted into the inner peripheralsurface of the tubular bearing holder portion 134 formed in the base 3.The shaft portion 35 b of the magnet holder 35 is a columnar member. Theshaft portion 35 b is press-fitted into the through-hole 32 of thesecondary shaft gear 30, and the lower portion of the shaft portion 35 bis inserted into inner rings of the two bearings 135. Accordingly, themagnet holder 35 is supported on the base 3 by the two bearings 135, androtates together with the secondary shaft gear 30. The magnet holder 35is rotatably held by the bearing holder portion 134 via the bearing 135about a rotation axis line parallel to the Z-axis.

Additionally, the magnet holding portion 35 a is provided at the upperend of the magnet holder 35. The magnet holding portion 35 a is abottomed cylindrical member. The magnet holding portion 35 a has adepression recessed from the upper end surface of the magnet holder 35toward the lower side. The inner peripheral surface of the depression inthe magnet holding portion 35 a is formed to be in contact with an outerperipheral surface Mqd of the magnet Mq. In the absolute encoder 2configured in this way, the magnet Mq is fixed to the magnet holdingportion 35 a by being accommodated in the depression of the magnetholding portion 35 a.

Because the shaft portion 35 b of the magnet holder 35 is supported bythe two bearings 135 disposed in the bearing holder portion 134 formedin the base 3, the magnet holder 35 can be prevented from tilting.Further, disposing the two bearings 135 at the furthest possibledistance away from each other in the up-down direction of the shaftportion 35 b increases the effect of preventing the magnet holder 35from tilting.

As illustrated in FIG. 16 , the magnet Mq is a disk-shaped orsubstantially disk-shaped permanent magnet to be press-fitted into themagnet holding portion 35 a of the magnet holder 35, and has the uppersurface Mqa and the lower surface Mqb. In the absolute encoder 2, theupper surface Mqa of the magnet Mq faces the lower surface of the anglesensor Sp with a certain distance in between. A central axis MqC of themagnet Mq (axis representing the center of the magnet Mq or axis passingthrough the center of a magnetic pole boundary) coincides with a centralaxis SC of the magnet holder 35, a central axis GsC of the secondaryshaft gear 30, and a central axis BC of the bearing 135. When thesecentral axes are made to coincide with each other, the rotation angle orthe amount of rotation can be detected with higher accuracy.

Note that in the embodiment of the present invention, the two magneticpoles (N/S) of the magnet Mq are preferably formed adjacent to eachother in a horizontal plane (XY plane) perpendicular to the central axisMqC of the magnet Mq. With this configuration, the detection accuracy ofthe rotation angle or the amount of rotation by the angle sensor Sq canbe further improved. Note that the magnet Mq is formed from a magneticmaterial such as a ferritic material, an Nd (neodymium)—Fe (iron)—B(boron) material. The magnet Mq may be, for example, a rubber magnet ora bond magnet including a resin binder.

In the absolute encoder 2, the main shaft gear 10, the intermediate gear20, and the secondary shaft gear 30 are provided as described above, andthe rotation axis lines of the main shaft gear 10 and the secondaryshaft gear 30 are parallel to each other. Further, the rotation axisline of the intermediate gear 20 is located at a twisted position withrespect to the rotation axis lines of the main shaft gear 10 and thesecondary shaft gear 30. By arranging each gear in this manner, theamount of rotation of the main shaft gear 10 over multiple rotations canbe identified according to the detection result of the angle sensor Sq.Because the rotation axis line of the intermediate gear 20 is located ata twisted position relative to the rotation axis lines of the main shaftgear 10 and the secondary shaft gear 30 and is orthogonal to therotation axis lines in a front view, the absolute encoder 2 can includea bent transmission path and be made thinner.

Next, the action of the magnetic interference reduction member 16 of theabsolute encoder 2 will be described.

In the absolute encoder 2, as described above, in the magnet Mp, thefirst magnetic pole portion N and the second magnetic pole portion S areprovided adjacent to each other in the radial direction with the centerof the magnet Mp in the radial direction as a boundary, and the firstmagnetic pole portion N and the second magnetic pole portion S areprovided adjacent to each other in the axial direction MpC with thecenter in the axial direction MpC as a boundary, and the magnet Mp ismagnetized in the plane direction. Therefore, in the magnet Mp, amagnetic flux other than the magnetic flux necessary for detecting therotation angle by the angle sensor Sp, that is, a leakage flux, affectsthe surroundings of another angle sensor or the like.

Here, in the absolute encoder 2, the annular magnetic interferencereduction member 16 formed of a magnetic member is provided on the outerperipheral surface Mpd of the magnet Mp in a manner surrounding theentire periphery of the outer peripheral surface Mpd. The innerperipheral surface 16 a of the magnetic interference reduction member 16is provided to be in contact with the outer peripheral surface Mpd ofthe magnet Mp. Because the magnetic interference reduction member 16 isprovided on the outer peripheral surface Mpd, the leakage flux from themagnet Mp toward the radially outer peripheral side is suppressed in theabsolute encoder 2. That is, in the absolute encoder 2, it is possibleto reduce magnetic interference on surroundings by the magnet Mp whilemaintaining the magnetic flux necessary for detecting the rotation angleby the angle sensor Sp.

Backlash Reduction Mechanism

As described above, the absolute encoder 2 includes the biasingmechanism 40 biasing the second worm gear portion 22 in the direction ofthe second worm wheel portion 31, and the biasing mechanism 40 is abacklash reduction mechanism configured to reduce backlash between thesecond worm gear portion 22 and the second worm wheel portion 31. Asillustrated in FIGS. 5, 6, 11, 14 , and others, the biasing mechanism 40includes the biasing spring 41, the support projection 45, and a screw 8d for fixing the biasing spring 41 to the support projection 45. Thethrough-hole 143 of the support projection 141 and the through-hole 145of the support projection 142 of the base 3 also constitute the biasingmechanism 40.

The biasing spring 41 is a member for generating a pressing forcepressing the second worm gear portion 22 in the direction of the secondworm wheel portion 31, and is an elastic member. The biasing spring 41is, for example, a leaf spring, and is formed of a metal plate. Asillustrated in FIGS. 12 and 14 , specifically, the biasing spring 41includes a spring portion 42 being a portion elastically deforming andgenerating a pressing force, and an engaging portion 43 and a fixingportion 44 being opposing portions sandwiching the spring portion 42.The engaging portion 43 and the fixing portion 44 are portions formingthe pair of end portions of the biasing spring 41.

The fixing portion 44 is formed to be fixed to the support projection 45projecting from the upper surface 104 of the base portion 101 of thebase 3 by the screw 8 d. The screw 8 d is an example of a fixing member.A hole 44 a receiving insertion of the screw 8 d is formed in the fixingportion 44. The fixing portion 44 extends in a planar shape and isconfigured to be fixed to the support projection 45 by the screw 8 dwhile in contact with a planar support surface 45 a of the supportprojection 45.

The engaging portion 43 has a shape capable of engaging with thesecondary shaft-side end portion 23 b of the intermediate gear shaft 23.The engaging portion 43 includes, for example, an engagement groove 43 aforming a gap extending along the extension direction from the springportion 42 of the engaging portion 43, as illustrated in FIGS. 13 and 14. The engagement groove 43 a is a groove open to a side of a tip endedge 43 b being an end edge opposing a connection portion 43 c of theengaging portion 43 with the spring portion 42, and is formed by abranched portion divided into two prongs, similar to the first end 9 aof the leaf spring 9 described above. The engaging portion 43 extends ina planar shape. An engagement-receiving groove 23 d being an annulargroove extending in a direction orthogonal or substantially orthogonalto the central axis of the intermediate gear shaft 23, is formed in thesecondary shaft-side end portion 23 b of the intermediate gear shaft 23.The engagement groove 43 a of the engaging portion 43 can engage withthe engagement-receiving groove 23 d. When one side of the engagementgroove 43 a parallel to the up-down direction presses the intermediategear shaft 23 in the engagement-receiving groove 23 d, the intermediategear 20 is biased in the direction of the second worm gear portion 22moving toward the second worm wheel portion 31. Additionally, two sidesof the engagement groove 43 a parallel to the left-right direction arein contact with the intermediate gear shaft 23 in theengagement-receiving groove 23 d, and movement of the biasing spring 41in the up-down direction is restricted by the intermediate gear shaft23.

The spring portion 42 has a shape being likely to elastically deform inthe engagement direction of the engaging portion 43 to the intermediategear shaft 23. Specifically, as illustrated in FIG. 14 , the springportion 42 has a shape likely to deflect in the extension direction ofthe engagement groove 43 a. For example, the spring portion 42 includesan arm portion 42 a extending in a planar shape from the connectionportion 43 c of the engaging portion 43, and a raised portion 42 bconnecting the arm portion 42 a and the fixing portion 44. The raisedportion 42 b is a portion extending obliquely from the fixing portion 44to one side facing the fixing portion 44.

The biasing spring 41 is fixed to the support projection 45 by the screw8 d at the fixing portion 44 in an orientation of the raised portion 42b being raised from the fixing portion 44 at the side opposite to thesupport projection 45. In this fixed state, the dimensions of the springportion 42 and the engaging portion 43, the angle between the extendingdirection of the spring portion 42 and the extending direction of theengaging portion 43, and the like are set such that the engagementgroove 43 a of the engaging portion 43 engages with theengagement-receiving groove 23 d of the intermediate gear shaft 23 and,in this engaged state, the spring portion 42 generates a pressing forcepressing the engaging portion 43 against the intermediate gear shaft 23.Also, in the fixed and engaged state of the biasing spring 41, the snapring 144 attached to the intermediate gear shaft 23 is in contact withthe outer side surface 141 a of the support projection 141. In order toreduce backlash as to be described below, the engagement groove 43 a ofthe engaging portion 43 is preferably formed extending in a directionorthogonal or substantially orthogonal to the central axis of theintermediate gear shaft 23 in the fixed state of the biasing spring 41.Note that the snap ring 144 can be omitted because the biasing spring 41can restrict movement of the intermediate gear shaft 23 in the centralaxial direction.

Next, the action of the biasing mechanism 40 of the absolute encoder 2will be described.

In the absolute encoder 2, the intermediate gear shaft 23 is supportedat the base 3 by the main shaft-side end portion 23 a being insertedinto the through-hole 143 formed in the support projection 141 of thebase 3 and the secondary shaft-side end portion 23 b being inserted intothe through-hole 145 formed in the support projection 142 of the base 3.Further, the snap ring 144 is attached to the groove 23 c of the mainshaft-side end portion 23 a inserted into the through-hole 143 in thesupport projection 141, and the snap ring 144 is attached to the groove23 c positioned at a side of the support projection 141 facing the outerside surface 141 a. In this manner, the intermediate gear shaft 23 issupported by the support projections 141 and 142 while movement from themain shaft-side end portion 23 a toward the secondary shaft-side endportion 23 b is restricted.

The intermediate gear 20 is thus rotatably supported by the intermediategear shaft 23. Furthermore, due to the action of the leaf spring 9, theintermediate gear 20 is biased toward the support projection 142, andthe secondary shaft-side sliding portion 26 of the intermediate gear 20abuts against an inner side surface 142 a of the support projection 142(see FIG. 13 ).

As described above, the through-hole 145 has a long hole shape with themajor axis longer than the minor axis and supports the secondaryshaft-side end portion 23 b of the intermediate gear shaft 23. Further,in the through-hole 145, the secondary shaft-side end portion 23 b issupported such that the secondary shaft-side end portion 23 b can movealong the major axis of the through-hole 145, that is, within the rangeof the major axis width of the through-hole 145 along with a horizontalplane. On the other hand, the through-hole 143 supporting the mainshaft-side end portion 23 a of the intermediate gear shaft 23 has around hole shape. Thus, in the absolute encoder 2, the intermediate gearshaft 23 can oscillate along a horizontal plane by the through-holes 143and 145 of the support projections 141 and 142 and the biasing mechanism40, with the supported portion of the main shaft-side end portion 23 aas a center or a substantial center.

Also, in the intermediate gear shaft 23 supported in this manner, theengaging portion 43 of the biasing spring 41 is engaged with theengagement-receiving groove 23 d of the secondary shaft-side end portion23 b, and the biasing spring 41 applies a biasing force to the secondaryshaft-side end portion 23 b of the intermediate gear shaft 23 to pressthe second worm gear portion 22 of the intermediate gear 20 toward thedirection of the second worm wheel portion 31 (second meshing directionP2) of the secondary shaft gear 30. As a result, the second worm gearportion 22 of the intermediate gear 20 is pressed against the secondworm wheel portion 31 of the secondary shaft gear 30, causing aso-called “bottoming-out” phenomenon occurs between the second worm gearportion 22 and the second worm wheel portion 31 such that the backlashbetween gears is zero.

Further, since the secondary shaft-side end portion 23 b at the movingside of the intermediate gear shaft 23, supported in an oscillatingmanner, is biased by the biasing spring 41, during oscillation, theintermediate gear shaft 23 is constantly biased in the direction of thesecond worm gear portion 22 moving toward the second worm wheel portion31. Therefore, the backlash between the second worm gear portion 22 andthe second worm wheel portion 31 can always be made zero without causingrotation malfunction between gears due to oscillation of theintermediate gear shaft 23.

For example, when the ambient temperature around the absolute encoder 2is high, the secondary shaft gear 30 expands according to the linearexpansion coefficient of the material, and the pitch circles of thegears of the second worm wheel portion 31 expand. At this time, if thethrough-hole 145 formed in the support projection 142 of the base 3 is around hole and not a long hole as in the present embodiment, thesecondary shaft-side end portion 23 b of the intermediate gear shaft 23is fixed by the through-hole 145, and the intermediate gear shaft 23cannot oscillate as in the present embodiment. Therefore, the secondworm wheel portion 31 of the secondary shaft gear 30, having expandedgear pitch circles due to the increase in temperature, may come intoforceful contact with the second worm gear portion 22 of 22 of theintermediate gear and the gear may not rotate.

Additionally, when the ambient temperature around the absolute encoder 2is low, the secondary shaft gear 30 contracts according to the linearexpansion coefficient of the material, and the pitch circles of thegears of the second worm wheel portion 31 decrease. At this time, if thethrough-hole 145 formed in the support projection 142 of the base 3 is around hole and not a long hole as in the present embodiment, thesecondary shaft-side end portion 23 b of the intermediate gear shaft 23is fixed by the through-hole 145, and the intermediate gear shaft 23cannot oscillate as in the present embodiment. In this case, thebacklash between the second worm gear portion 22 of 22 of theintermediate gear and the second worm wheel portion 31 of the secondaryshaft gear 30 increases, and the rotation of the 22 of the intermediategear is not accurately transferred to the secondary shaft gear 30.

In contrast, in the absolute encoder 2 according to the presentembodiment, as described above, the intermediate gear shaft 23 issupported in a manner allowing the intermediate gear shaft 23 tooscillate along a horizontal plane with the supported portion of themain shaft-side end portion 23 a as a center or a substantial center,and the intermediate gear 20 is constantly biased from the second wormgear portion 22 side to the second worm wheel portion 31 side by thebiasing mechanism 40. Additionally, the intermediate gear 20 supportedby the intermediate gear shaft 23 is biased toward the supportprojection 142 by the leaf spring 9. Therefore, even when a change inthe ambient temperature occurs and the pitch circles of the gears of thesecond worm wheel portion 31 of the secondary shaft gear 30 change asdescribed above, the backlash becomes zero while the tooth surfacesbetween the second worm gear portion 22 and the second worm wheelportion 31 are kept in contact by an appropriate pressing force.Therefore, it is possible to avoid non-rotation of the gear due to achange in temperature and deterioration of the accuracy of the rotationtransmitted from the intermediate gear 20 to the secondary shaft gear30.

Note that, regardless of the position of the secondary shaft-side endportion 23 b of the intermediate gear shaft 23 due to oscillation, thebiasing mechanism 40 is preferably set such that a constant orsubstantially constant pressing force is generated from the biasingspring 41.

As described above, the through-hole 143 of the support projection 141supporting the main shaft-side end portion 23 a of the intermediate gearshaft 23 has a round hole shape, the through-hole 145 of the supportprojection 142 supporting the secondary shaft-side end portion 23 b hasa long hole shape with the major axis-side width larger than the minoraxis-side width, and the intermediate gear shaft 23 can oscillate inparallel or substantially parallel with the horizontal direction withthe through-hole 143 of the support projection 141 as a fulcrum.Therefore, during oscillation of the intermediate gear shaft 23, theamount of movement of the second worm gear portion 22 relative to thesecond worm wheel portion 31 is greater than the amount of movement ofthe first worm wheel portion 21 relative to the first worm gear portion11, and the first worm gear portion 11 and the first worm wheel portion21 do not bottom out even if the second worm gear portion 22 and thesecond worm wheel portion 31 bottom out.

As illustrated in FIGS. 11 and 12 , the through-hole 145 supporting theintermediate gear shaft 23 at the secondary shaft-side end portion 23 bforms a tubular surface or a substantially cylindrical surface, but thethrough-hole 145 is not limited to having such a shape. For example, asillustrated in FIG. 17 , the cross-sectional shape of the through-hole145 may be a rectangle or a substantial rectangle instead of a longhole. In other words, the through-hole 145 may be a through-holeextending in a quadrangular pillar shape and forming a pair of surfaces145 a opposing each other and a pair of surfaces 145 b opposing eachother. The pair of surfaces 145 a and the pair of surfaces 145 b formingthe through-hole 145 may be flat surfaces or curved surfaces. In theabsolute encoder 2, the pair of surfaces 145 a extend in the horizontaldirection, and the pair of surfaces 145 b extend in the up-downdirection. The width in the up-down direction of the surface 145 a isgreater than the width in the vertical direction of the surface 145 b.The intermediate gear shaft 23 can oscillate in the through-hole 145illustrated in FIG. 17 , similarly to the through-hole 145 describedabove.

Similarly, the through-hole 143 is not limited to having the shapedescribed above. For example, the through-hole 143 may have a so-calledknife edge structure. More specifically, the through-hole 143 may be incontact with the intermediate gear shaft 23 by line contact or pointcontact. For example, as illustrated in (a) and (b) of FIG. 18 , thethrough-hole 143 may be formed by a pair of conical or substantiallyconical inclined surfaces 143 c having a smaller diameter further inwardalong the extension direction of the through-hole 143. In this case, thethrough-hole 143 is in contact with and supports the intermediate gearshaft 23 along a closed line (connection line 143 d) depicting a roundhole of the connection portions of the pair of inclined surfaces 143 c.The round hole shape of the connection line 143 d has a similar shape ina plan view to the round hole shape of the through-hole 143 describedabove. Since the through-hole 143 supports the intermediate gear shaft23 by line contact or point contact, the intermediate gear shaft 23 canbe oscillate even if the diameter of the round hole of the through-hole143 is made closer to the diameter of the intermediate gear shaft 23.Thus, the cross-sectional shape of the through-hole 143 can be madecloser to a shape having no gap between the through-hole 143 and theintermediate gear shaft 23. Such a configuration can suppress movementof the portion of the intermediate gear shaft 23 in contact with thethrough-hole 143 during oscillation of the intermediate gear shaft 23,and the oscillation of the intermediate gear shaft 23 can suppressvariation in the distance between the first worm gear portion 11 and thefirst worm wheel portion 21. Note that the through-hole 145 of thesupport projection 142 may also have the so-called knife edge structurelike the through-hole 143 of the support projection 141 described above,or may be formed by a pair of conical or substantially conical inclinedsurfaces forming a closed line depicting a long hole.

As illustrated in (a) and (b) of FIG. 19 , the through-hole 145 may beformed by a pair of quadrangular pyramid-shaped or substantiallyquadrangular pyramid-shaped inclined surfaces 145 e tapering toward theinner side in the extension direction of the through-hole 145. In thiscase, the through-hole 145 is in contact with and supports theintermediate gear shaft 23 along a closed line (connection line 145 f)depicting a rectangular shape or substantially rectangular shape of theconnection portions of the pair of inclined surfaces 145 e. Theconnection line 145 f has line portions 145 g being a pair of portionsopposing each other, and line portions 145 h being a pair of portionsopposing each other. The pair of line portions 145 g and the pair ofline portions 145 h may be straight lines or curved lines. In theabsolute encoder 2, the pair of line portions 145 g extend horizontally,and the pair of line portions 145 h extend in the up-down direction. Thelength of the line portion 145 g is greater than the length of the lineportion 145 h in the up-down direction. Note that the through-hole 143of the support projection 141 may also be formed by a pair ofquadrangular pyramid-shaped or substantially quadrangular pyramid-shapedinclined surfaces forming a closed line depicting a rectangular shape ora substantially rectangular shape, similar to the through-hole 145 ofthe support projection 142 described above. In this case, the closedline is a square or a substantial square. In this case as well, similarto the case in FIG. 18 described above, since the through-hole 143supports the intermediate gear shaft 23 by line contact or pointcontact, the intermediate gear shaft 23 can oscillate even if the lengthof the line portion extending in the up-down direction (corresponding tothe line portion 145 h in FIG. 19 ) and the length of the line portionextending in the horizontal direction (corresponding to the line portion145 g in FIG. 19 ) are made closer to the diameter of the intermediategear shaft 23. Thus, the shape of the through-hole 143 can be madecloser to a shape having no gap in the up-down direction and thehorizontal direction between the through-hole 143 and the intermediategear shaft 23. Such a configuration can suppress movement of the portionof the intermediate gear shaft 23 in contact with the through-hole 143during oscillation of the intermediate gear shaft 23, and theoscillation of the intermediate gear shaft 23 can suppress variation inthe distance between the first worm gear portion 11 and the first wormwheel portion 21.

Control Unit

Next, a control unit of the absolute encoder 2 will be described. FIG.20 is a view of the substrate 5 in FIG. 2 as viewed from the lowersurface 5 a side. The microcomputer 51, a line driver 52, abidirectional driver 53, and the connector 6 are mounted on thesubstrate 5. The microcomputer 51, the line driver 52, the bidirectionaldriver 53, and the connector 6 are electrically connected by patternwiring on the substrate 5.

The bidirectional driver 53 performs bidirectional communication with anexternal device connected to the connector 6. The bidirectional driver53 converts data such as operation signals into differential signals tocommunicate with the external device. The line driver 52 converts datarepresenting the amount of rotation into a differential signal, andoutputs the differential signal in real time to the external deviceconnected to the connector 6. The connector 6 is connected to aconnector of the external device.

FIG. 21 is a block diagram schematically illustrating a functionalconfiguration of the absolute encoder 2 in FIG. 1 . Each block of themicrocomputer 51 illustrated in FIG. 21 represents a function realizedby executing a program by using a central processing unit (CPU) servingas the microcomputer 51.

The microcomputer 51 includes a rotation angle acquisition unit 51 p, arotation angle acquisition unit 51 q, a table processing unit 51 b, arotation amount identification unit 51 c, and an output unit 51 e. Therotation angle acquisition unit 51 p acquires a rotation angle Ap of themain shaft gear 10 based on a signal output from the angle sensor Sp.The rotation angle Ap is angle information indicating the rotation angleof the main shaft gear 10. The rotation angle acquisition unit 51 qacquires a rotation angle Aq of the secondary shaft gear 30 based on asignal output from the magnetic sensor Sq. The rotation angle Aq isangle information indicating the rotation angle of the secondary shaftgear 30. The table processing unit 51 b refers to a correspondence tablestoring the rotation angle Aq of the secondary shaft gear 30 and therotation speed of the main shaft gear 10 corresponding to the rotationangle Aq of the secondary shaft gear 30 to identify the rotation speedof the main shaft gear 10 corresponding to the acquired rotation angleAq of the secondary shaft gear 30. The rotation amount identificationunit 51 c identifies the amount of rotation of the main shaft gear 10over multiple rotations according to the rotation speed of the mainshaft gear 10 identified by the table processing unit 51 b and theacquired rotation angle Ap of the main shaft gear 10. The output unit 51e converts the identified amount of rotation of the main shaft gear 10over multiple rotations to information indicating the amount of rotationand outputs the information.

As described above, in the absolute encoder 2 according to the presentembodiment, the backlash between the second worm gear portion 22 of theintermediate gear 20 and the second worm wheel portion 31 of thesecondary shaft gear 30 can be made zero due to the action of thethrough-holes 143 and 145 supporting the intermediate gear shaft 23 andthe biasing spring 41. Furthermore, even when the ambient temperaturechanges, the backlash can be made zero while the teeth surfaces of thesecond worm gear portion 22 of the intermediate gear 20 and the secondworm wheel portion 31 of the secondary shaft gear 30 are always incontact at an appropriate pressing force by the action of the biasingspring 41 with respect to the intermediate gear shaft 23, supported in amanner allowing oscillation. Therefore, it is possible to avoidnon-rotation of the gears due to a change in temperature anddeterioration of the accuracy of the rotation transmitted from theintermediate gear 20 to the secondary shaft gear 30.

In this way, according to the absolute encoder 2 according to thepresent embodiment, the influence of backlash in a reduction mechanismon detection accuracy can be reduced. As a result, it is possible towiden the range of the identifiable amount of rotation of the main shaft1 a while maintaining the identifiable resolution of the amount ofrotation of the main shaft 1 a.

In addition, in the absolute encoder 2 according to the presentembodiment, the intermediate gear 20 disposed along the horizontal planeis provided to extend obliquely with respect to the outer peripheralsurfaces 105 to 108 of the base 3. Thus, the dimensions of the absoluteencoder 2 in the front-rear direction and the left-right direction canbe reduced.

Additionally, in the absolute encoder 2 according to the presentembodiment, the outer diameters of the first worm wheel portion 21 andthe second worm wheel portion 31 and the outer diameters of the firstworm gear portion 11 and the second worm gear portion 22 are set to thesmallest possible value. This makes it possible to reduce the dimensionsof the absolute encoder 2 in the up-down direction (height direction).

An embodiment of the present invention has been described above, but thepresent invention is not limited to the absolute encoder 2 according tothe embodiment of the present invention described above, and includesvarious aspects included in the gist of the present invention and thescope of the claims. Further, configurations may be combined with eachother or combined with known technology as appropriate to at leastpartially address the problem described above and achieve the effectsdescribed above. For example, a shape, a material, an arrangement, asize, and the like of each of the components in the embodiment describedabove may be changed as appropriate according to a specific usage aspectof the present invention.

For example, in the absolute encoder 2 according to the presentembodiment, the magnetic interference reduction members 16 and 17 areprovided on the outer peripheral surfaces Mpd and Mqd of the magnet Mpfixed to the upper surface of the main shaft gear 10 such that thecentral axes of both coincide or substantially coincide and the magnetMq fixed to the upper surface of the secondary shaft gear 30 such thatthe central axes of both coincide or substantially coincide, but thepresent invention is not limited to this configuration. That is, in theabsolute encoder 2, the magnetic interference reduction member need onlybe provided on at least one of the outer peripheral surface Mpd and Mqdof the magnet Mp fixed to the upper surface of the main shaft gear 10such that the central axes of both coincide or substantially coincide orthe magnet Mq fixed to the upper surface of the secondary shaft gear 30such that the central axes of both coincide or substantially coincide.In addition, for example, the shapes of the magnetic interferencereduction members 16 and 17 are not limited to the above-describedshapes, and the magnetic interference reduction members 16 and 17 needonly be provided on the outer peripheral surfaces Mpd and Mqd of themagnets Mp and Mq and reduce the leakage flux.

REFERENCE SIGNS LIST

1 Motor

1 a Main shaft

1 b Press-fitting section

2 Absolute encoder

3 Base

4 Case

4 a Outer wall portion

4 b Lid portion

4 c Hook portion

5 Substrate

5 a Lower surface

5 b Positioning hole

6 Connector

8 a, 8 b, 8 c, 8 d Screw

9 Leaf spring

9 a First end

9 b Second end

10 Main shaft gear

11 First worm gear portion

12 Main shaft adapter

12 a Upper end surface

13 Tubular portion

13 a Upper end surface

14 Press-fitting section

15 Magnet holding portion

15 a Inner peripheral surface

15 b Bottom surface

16 Magnetic interference reduction member

16 a Inner peripheral surface

16 b Outer peripheral surface

17 Magnetic interference reduction member

17 a Inner peripheral surface

17 b Outer peripheral surface

20 Intermediate gear

21 First worm wheel portion

22 Second worm gear portion

23 Intermediate gear shaft

23 a Main shaft-side end portion

23 b Secondary shaft-side end portion

23 c Groove

23 d Engagement-receiving groove

24 Tubular portion

24 a Through-hole

24 b Inner peripheral surface

25 Main shaft-side sliding portion

26 Secondary shaft-side sliding portion

30 Secondary shaft gear

31 Second worm wheel portion

32 Through-hole

35 Magnet holder

35 a Magnet holding portion

35 b Shaft portion

40 Biasing mechanism

41 Biasing spring

42 Spring portion

42 a Arm portion

42 b Raised portion

43 Engaging portion

43 a Engagement groove

43 b Tip end edge

43 c Connection portion

44 Fixing portion

44 a Hole

45 Support projection

45 a Support surface

51 Microcomputer

51 b Table processing unit

51 c Rotation amount identification unit

51 e Output unit

51 p, 51 q Rotation angle acquisition unit

52 Line driver

53 Bidirectional driver

101 Base portion

102 Lower surface

103 Recess part

104 Upper surface

105 to 108 Outer peripheral surface

110 Substrate pillar

111 Upper end surface

112 Screw hole

120 Substrate positioning pin

121 Tip end portion

122 Base portion

123 Stepped surface

131, 132, 141, 142 Support projection

131 a Screw hole

134 Bearing holder portion

135 Bearing

141 a Outer side surface

142 a Inner side surface

143, 145 Through-hole

145 a, 145 b Surface

143 c, 145 e Inclined surface

143 d, 145 f Connection line

145 g, 145 h Line portion

144 Snap ring

Ap, Aq Angle information

BC Center of bearing

GmC Central axis of main shaft gear

GsC Central axis of secondary shaft gear

MoC Central axis of main shaft of motor

Mp, Mq Magnet

Mpa, Mqa Upper surface

Mpb, Mqb Lower surface

Mpd, Mqd Outer peripheral surface

MpC, MqC Central axis of magnet

P Biasing direction

P1 First meshing direction

P2 Second meshing direction

R1 First transmission mechanism

R2 Second transmission mechanism

SaC Central axis of main shaft adapter

SC Central axis of magnet holder

Sp, Sq Angle sensor

XYZ Cartesian coordinate system

1. An absolute encoder configured to identify an amount of rotation of amain shaft over multiple rotations, the absolute encoder comprising: afirst drive gear configured to rotate according to rotation of the mainshaft; a first driven gear configured to mesh with the first drive gear;a second drive gear provided coaxially with the first driven gear andconfigured to rotate according to rotation of the first driven gear; asecond driven gear configured to mesh with the second drive gear; and amagnet provided at a tip end side of at least one of the first drivengear and the second driven gear, and an angle sensor configured todetect a rotation angle of the first driven gear or the second drivengear provided with the magnet in response to a change in a magnetic fluxgenerated from the magnet, wherein a first magnetic pole portion of afirst polarity and a second magnetic pole portion of a second polaritydifferent from the first polarity are formed adjacent to each other inthe magnet as viewed from an axial end surface of the magnet, the firstmagnetic pole portion and the second magnetic pole portion are formedadjacent to each other in a radial direction with a center of the magnetin the radial direction as a boundary, the first magnetic pole portionand the second magnetic pole portion are formed adjacent to each otherin the axial direction with a center in the axial direction as aboundary, and a magnetic interference reduction member formed of amagnetic material is provided on an outer peripheral surface of themagnet in the radial direction.
 2. The absolute encoder according toclaim 1, wherein the magnetic interference reduction member is providedsurrounding an entire outer peripheral surface of the magnet in theradial direction.
 3. The absolute encoder according to claim 1, whereinthe magnetic interference reduction member is in contact with an outerperipheral surface of the magnet in the radial direction.
 4. Theabsolute encoder according to claim 1, wherein the magnet is provided ata tip end side of the second driven gear, and the angle sensor detects arotation angle of the second driven gear.
 5. The absolute encoderaccording to claim 1, wherein the magnet is provided at a tip end sideof the first driven gear, and the angle sensor detects a rotation angleof the first driven gear.